Semiconductor manufacturing technology is driven by digital circuits such as microcontrollers, digital signal processors, memories, etc. The systems which use these circuits are getting more sophisticated and complex, their performance is increasingly enhanced and more features are being added, requiring more transistors to be integrated on a chip and higher computing power. To accommodate these needs while lowering costs, semiconductor device manufacturers have introduced ever smaller geometries in their semiconductor process to reduce transistor size and squeeze more transistors on a single silicon wafer.
The smaller geometries require a corresponding reduction in the supply voltage for these circuits. However, the rechargeable battery voltage has remained largely unchanged. For example, the supply voltage of the currently available 90 nm and 65 nm standard CMOS processes has dropped to 1.2V, whereas the voltage of a fully charged lithium ion battery pack remains up to 5.0V. This incompatibility in supply voltage causes problems in system design and prevents microcontrollers from being powered directly by battery. The approach to circumvent this dilemma is to use a dedicated switching or voltage regulator to convert the battery voltage to the required supply voltage for the microcontroller.
Power switches are key component in designing switching regulators. As these power switches are turned on and off, DC-DC conversion is performed with high power efficiency. FIG. 1A shows a circuit diagram of a basic step-down switching regulator (10). The circuit includes a high-side power switch (HSPS), low-side power switch (LSPS), inductor (L), capacitor (C), and controller and driver circuit (12). The high-side power switch may be a p-type transistor and low-side power switch an n-type transistor.
One conversion cycle consists of two modes of operation. First, HSPS is turned on and LSPS is off, so that battery voltage (Vbat) is applied at the left terminal of the inductor and the current flowing through the inductor increases. Then HSPS is turned off. At the same time, LSPS is turned on to provide a path for the inductor current which is now decreasing. The inductor and capacitor form a low-pass filter so that the converted low voltage over the load (Vload) remains fairly constant except for a small ripple. This process repeats cycle by cycle. The voltage at the midpoint where the two power switches HSPS and LSPS are connected (VM) is also shown in FIG. 1A. FIG. 1B shows the variation in the midpoint voltage (VM) for several cycles of the switching regulator. Both power switches must be able to withstand a voltage up to the input voltage to the switching regulator, i.e. the battery voltage (Vbat).
Currently, the most widely used battery type for portable or hand-held electronic devices such as cellular phones and MP3 players, is the lithium ion battery. Typically, a fully charged lithium ion battery pack reaches a voltage of up to 5V. As the battery discharges its voltage decreases, with the lowest allowed voltage being about 3.0V. Clearly, the two power switches in the switching regulator of FIG. 1A must withstand 5V. However, the maximum allowed voltage for CMOS transistors in the current state-of-the-art 65 nm process technology is only 2.5V.
Currently, regulators are available as stand alone integrated circuits and their manufacturing entails a high-voltage semiconductor process. Such processes include the bipolar and bipolar CMOS (BiCMOS) processes. In addition, various other high-voltage CMOS processes have been developed, but they are generally more expensive to use than standard CMOS processes, and are not used for microprocessors or DSPs. Another conventional solution has been to use MOS transistors with an extended drain. However, such devices with an extended drain have to be characterized before they can be utilized. Another drawback is that the extended drain only permits the drain-to-source voltage to be increased, whereas the maximum gate-to-source and gate-to-drain voltages are not affected and remain a problem.
The invention seeks to address these problems by providing a method of realizing high-voltage power switches using transistors made using a standard 65 nm CMOS process. These proposed switches can also be used in class-D audio power amplifiers.